Photodetector using nanoparticles

ABSTRACT

The present invention relates to a photodetector using nanoparticles, and more particularly, to a novel photodetector wherein surfaces of nanoparticles synthesized by a wet colloidal process are capped with organic materials which then serve as channels for electron migration, or nanoparticles, from which organic materials capped on the surfaces of nanoparticles are removed to form a close-packed particle structure, directly serve to transport electrons. In accordance with specific embodiments of the present invention, it is possible to improve performance of the photodetector and simplify the manufacturing process thereof.

BACKGROUND OF THE INVENTION

This application is a continuation of application Ser. No. 11/102,930filed Apr. 11, 2005, which claims priority under 35 U.S.C. §119(a) toKorean Patent Application No. 2004-91898 filed on Nov. 11, 2004, both ofwhich are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a photodetector using nanoparticles.More particularly, the present invention relates to a photodetectorusing nanoparticles with organic material-capped surfaces synthesized bya wet colloidal process, or close-packed nanoparticles formed by removalof surface-capped organic materials.

DESCRIPTION OF THE RELATED ART

As a conventional photodetector 10 using quantum effects, a structuretaking advantage of quantum well (QW) effects is well known to thoseskilled in the art, as shown in FIG. 1. However, where light isirradiated in the direction vertical to QW layer 11 (direction a),optical pumping may occur (due to influence of an electrical fieldpresent in the vertical direction) relative to the light direction(i.e., in the direction horizontal to the QW layer 11) and as a result,high quantum efficiency is not obtained in the planar direction(direction a), which is present in a bulk form. Conversely, where lightis irradiated in the direction horizontal to QW layer 11 (direction b),even though optical pumping may occur due to effects of quantum effects,it is difficult to collect light on a QW region that is physicallyseveral nanometers in size, thereby resulting in poor light receivingefficiency. In addition, the resulting photocurrent flows in thefollowing route: quantum well (QW) layer 11→quantum barrier (QB)12→quantum well (QW) layer 13, as shown in FIG. 2, thereby leading tolowered efficiency. Where it is desired to detect a wavelength in a farinfrared region using such a conventional photodetector, several hundredQW layers 11, serving as channels, are required, thus necessitating aprolonged period of time and highly expensive equipment in fabricatingthe photodetector.

In the case of the photodetector made up of quantum dots, as hasrecently been reported, the quantum dots self-assemble when prepared byvapor phase methods such as Metal Organic Chemical Vapor Deposition(MOCVD) and Molecular Beam Epitaxy (MBE). The basic structure of thequantum dot photodetector synthesized by such vapor phase methods takesa form in which self-assembled quantum dots are incorporated into aquantum well active layer in a conventional quantum well photodetectorp-n structure. In this connection, the photodetector having such astructure suffers from the following problems:

1) Since quantum dots are not connected to one another and thusindependently present due to characteristics inherent to synthesismethods used, an existing quantum barrier makes it difficult toefficiently generate photocurrent even when light is received andelectrons are activated.

2) Since self-assembled quantum dots, which are prepared by MOCVD orMBE, have inevitably a barrier layer after formation of a monolayer, thethickness of a light-receiving region is too small, which in turnresults in light scattering and very low efficiency of receiving light.In particular, when it is desired to receive infrared light, the activelayer should have a thickness of several nanometers, but it is not easyto achieve this thickness.

3) Finally, the p-n structure entails complicated fabrication processesand poor efficiency of a light-receiving window.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblems, and it is an object of the present invention to provide aphotodetector capable of improving performance thereof and simplifying amanufacturing process, by using nanoparticles with organicmaterial-capped or coated surfaces synthesized by a wet colloidalprocess, or close-packed nanoparticles formed by removal ofsurface-capped organic materials.

In accordance with an aspect of the present invention, the above andother objects can be accomplished by the provision of a photodetector,comprising a substrate; an insulating layer formed on the substrate; andtwo opposite electrodes disposed on the insulating layer, at apredetermined distance, wherein a nanocrystalline layer includingnanoparticles with organic material-capped surfaces prepared by a wetcolloidal synthesis method is formed between two opposite electrodes.

In accordance with another aspect of the present invention, there isprovided a photodetector, comprising a substrate; an insulating layerformed on the substrate; and two opposite electrodes disposed on theinsulating layer, at a predetermined distance, wherein a nanocrystallinelayer including nanoparticles prepared by a wet colloidal synthesismethod and close-packed by removal of surface-capped organic materialsis formed between two opposite electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description of exemplary embodiments taken in conjunction withthe accompanying drawings, in which:

FIG. 1 shows a cross-sectional view of a photodetector using quantumeffects in accordance with a conventional art;

FIG. 2 schematically shows an operational principle of a photodetectorusing quantum effects in accordance with a conventional art;

FIG. 3 shows a schematic cross-sectional view of a photodetector usingnanoparticles in accordance with one embodiment of the presentinvention;

FIG. 4 shows migration of holes through organic channels in aphotodetector using nanoparticles in accordance with one embodiment ofthe present invention;

FIG. 5 shows migration of electrons through close-packed nanoparticles,in a photodetector using nanoparticles in accordance with one embodimentof the present invention;

FIG. 6 shows energy state density with respect to particle size ofnanoparticles;

FIG. 7 shows an electron micrograph of HgTe nanoparticles obtained inPreparation Example 1 of the present invention;

FIG. 8 shows photoluminescence spectra of HgTe nanoparticles obtained inPreparation Example 1 of the present invention, in solution state andthin film state thereof; and

FIGS. 9 and 10 show changes in photocurrent degree of nanoparticles usedin a photodetector in accordance with one embodiment of the presentinvention, with respect to the passage of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in more detail withreference to accompanying drawings.

FIG. 3 shows a structure of a photodetector using nanoparticles inaccordance with one embodiment of the present invention. Referring toFIG. 3, the photodetector 30 in accordance with the present inventionhas a structure in which an insulating layer 32 (e.g., SiO₂) is formedon a substrate 31, two opposite electrodes 33 a, 33 b are disposed onthe insulating layer 32 at a predetermined distance and ananocrystalline layer 34 composed of nanoparticles is disposed betweentwo electrodes 33 a, 33 b, made of, for example, Au.

The photodetector 30 in accordance with the present invention havingsuch a structure is synthesized by a wet colloidal process and comprisesnanoparticles having surfaces capped with organic materials.Nanoparticles utilized in the present invention may be prepared byconventionally known wet colloidal synthesis methods and may be obtainedby placing nanoparticle materials and organic materials for cappingsurfaces of nanoparticles in a solvent, and then reacting the mixtureaccording to a wet colloidal synthesis method to prepare nanoparticleshaving surfaces capped with organic materials.

Using the wet colloidal synthesis method, it is advantageously possibleto obtain nanoparticles exhibiting excellent crystallinity and it isrelatively easy to control particle size and distribution, as comparedto synthesis methods using MBE or MOCVD, and it is also possible toprepare particles having high surface density. In addition, there areprovided advantages such as very low production costs, mass productionand high competitiveness from the standpoint of commercialization in thenear future, as compared to MBE or MOCVD.

FIG. 4 shows migration of holes through organic channels in aphotodetector 30 using the thus-prepared nanoparticles 34 a inaccordance with this exemplary embodiment of the present invention. Thatis, light irradiation to electrons or holes present in valance bands ofnanoparticles induces excitation of conduction bands, and excitedelectrons or holes migrate through the capped organic material,1-thioglycerol, for example, thereby resulting in current flow.

Meanwhile, the photodetector in accordance with a second embodiment ofthe present invention comprises close-packed nanoparticles 34 b formedby removing organic material capped on the surface of nanoparticles. Asdescribed above, nanoparticles having surfaces capped with an organicmaterial were prepared using the conventional wet colloidal synthesismethod and then washed with acetone and ethanol to obtain nanoparticlesfrom which the capped organic material had been removed.

FIG. 5 shows electron migration through close-packed nanoparticles 34 bin the photodetector using the thus-prepared nanoparticles in accordancewith this exemplary embodiment of the present invention. That is,because the organic material capped on the surfaces of the nanoparticles34 a was removed, electrons excited by light irradiated upon thenanoparticles 34 b directly migrate through nanoparticles, therebyresulting in current flow.

As nanocrystals 34 that can be utilized in exemplary embodiments of thephotodetector of the present invention, mention may be made ofsemiconductor nanoparticles selected from the group consisting of HgTe,HgSe, HgS, CdTe, CdSe, CdS, ZnTe, ZnSe, ZnS, PbTe, PbSe, PbS andmixtures thereof. Where nanocrystals are in the form of mixtures, theymay be present in the form of a simple mixture, or crystal structures ofthe respective materials may be partially divided and present in thesame particles, or in the form of alloys. Preferably, a nanocrystallinelayer composed of nanocrystals in accordance with the present inventionhas a thickness ranging from several hundreds of nm to several tens ofmicrons.

Among above-mentioned materials, a great deal of attention has beenparticularly directed to HgTe, because an energy gap of bulk HgTe isalmost 0 eV. By controlling a particle size to the range of from 3 nm to10 nm, HgTe nanoparticles will have the energy gap covering the entireinfrared wavelength region ranging from 0.9 μm to several tens of μm.

As organic materials that are capped on surfaces of nanoparticles toserve as channels for electron migration, in the photodetector inaccordance with the present invention, they include, but are not limitedto, alkyldithiol, mercaptoalkyl alcohol, mercaptoalkyl acid and mixturesthereof.

As specific examples of organic materials, mention may be made ofalkyldithiols such as 1-thioglycerol, hexanedithiol, octanedithiol,nonanedithiol and dodecanedithiol; mercaptoalkyl alcohols such asmercaptohexanol and mercaptooctanol; and mercaptoalkyl acids such asmercapto acetic acid and mercapto propionic acid.

Nanoparticles that can be utilized in the present invention includenanoparticles in the form of both single structure and core-shellstructures.

Meanwhile, control of the nanoparticle size via use of the wet colloidalsynthesis method makes it possible to prepare an optical element capableof selectively receiving light with a particular wavelength within theoverall infrared wavelength region. FIG. 6 shows energy state densitywith respect to particle size of nanoparticles. Controlling nanoparticlesize results in control of band gap C, and thereby it is possible tofabricate a photodetector adapted for receiving light with a desiredwavelength and a high speed and high efficiency photodetector. Thesmaller the nanoparticle size, the larger the band gap.

As the substrate for use in the photodetector in accordance with thepresent invention, conventional substrates may be employed.Specifically, mention may be made of silicon, alumina and glass, asexamples.

As materials for the insulating layer used in the photodetector inaccordance with the present invention, any material that isconventionally used may be employed. Specifically, mention may be madeof SiO₂, Al₂O₃, Indium Tin Oxide and HfO, as example. Preferably, theinsulating layer has a thickness of more than 100 μm.

As electrodes used in the photodetector in accordance with the presentinvention, any material that is conventionally used may be employed.Specifically, mention may be made of Au, Al, Pt and Cu, as examples.

EXAMPLES

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Preparation Example 1 Preparation of Organic Material-CappedNanoparticles

0.94 g (2.35 mmol) of Hg(ClO₄)₂)3H₂O (Aldrich) and 0.5 mL (5.77 mmol) of1-thioglycerol (Aldrich) were added to 125 mL of ultra pure water. 1MNaOH was then added dropwise thereto to adjust the pH of the resultingsolution to 11.63. This solution was charged to a three-necked flask andwas bubbled for 30 min under nitrogen gas atmosphere. Next, 0.028 g ofAl₂Te₃ (Cerac) and 10 mL of 0.5M H₂SO₄ were reacted to generate H₂Tegas, and the resulting H₂Te gas and nitrogen gas were passed through thesolution. The solution was stirred to facilitate initiation of reactionin the flask and evacuated to vacuum. After reaction, a solvent wasremoved from the resulting dark brown solution and concentrated to about30 mL. The concentrated solution was centrifuged to obtain nanoparticlepowder. The crystal structure of the thus-prepared nanoparticles wasexamined under an electron microscope. The results are shown in FIG. 7.In addition, FIG. 8 shows photoluminescence spectra of nanoparticles insolution state and film state.

Example 1 Photodetector Containing Nanoparticles Using Organic Channels

On a silicon substrate on which an insulating layer was formed in thethickness of 300 nm, Au electrodes having a horizontal structure andspaced at a distance of 5 μm relative to each other, fabricated using aphotolithography process, were disposed. A solution of HgTenanoparticles prepared in Preparation Example 1 was re-dispersed inwater and the resulting solution was dropped between Au electrodes,followed by drying to prepare a film having a thickness of 10 μm.

The thus-prepared HgTe nanoparticle film was irradiated with a lightsource having a wavelength of 1.1 μm, which is within the near-infraredregion, using a Quartz-Tungsten-Halogen (QTH) lamp and a spectroscope,and a current of 3 volts was applied to the electrodes. This wasfollowed by measurement of photocurrent. The results are shown in FIG.9.

Example 2 Photodetector Containing Organic Material-RemovedNanoparticles

HgTe nanoparticles prepared in Preparation Example 1 were washed withacetone three times for one minute each time and with ethanol once, soas to remove 1-thioglycerol around nanoparticles, followed by drying.The organic material-removed nanoparticles thus obtained were droppedbetween Au electrodes having a horizontal structure and spaced at adistance of 5 μm relative to each other, fabricated using aphotolithography process, on a silicon substrate having a 300 nminsulating layer formed thereon, and then was dried to prepare a filmhaving a thickness of 10 μm.

The thus-prepared HgTe nanoparticle film was irradiated with a lightsource having a wavelength of 1.1 μm, which is within the near-infraredregion, using a Quartz-Tungsten-Halogen (QTH) lamp and a spectroscope,and a current of 3 volts was applied to the electrodes. This wasfollowed by measurement of photocurrent. The results are shown in FIG.10.

As described above, since the photodetector in accordance with thepresent invention uses nanoparticles formed by wet chemical methods, itis possible to effect economical and mass production of thephotodetector. In addition, since current carriers can migrate throughthe organic material capped on the nanoparticle surface or organicmaterial-removed nanoparticles, it is possible to simplify thephotodetector structure and manufacturing process. Further, utilizationof isotropic quantum effects of nanoparticles enables fabrication ofhigh-efficiency photodetectors.

Also, when the photodetector is fabricated using nanoparticles inaccordance with the present invention, wavelengths that are desired toreceive and detect are variable over the entire infrared region and thusthe photodetector is applicable to a very wide area such as applicationto infrared detection devices for optical communication and militarypurpose.

Although the preferred embodiments of the present disclosure have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method of synthesizing a photodetector comprising: forming a substrate; forming an insulating layer formed on the substrate; forming two opposite electrodes disposed on the insulating layer at a predetermined distance; and forming a nanocrystalline layer comprising close-packed nanoparticles formed between two opposite electrodes, wherein a method of synthesizing the close-packed nanoparticles is according to a wet colloidal synthesis method and removal of surface-capped with organic materials.
 2. The method according to claim 1, wherein the method of synthesizing the close-packed nanoparticles further comprises concentrating a solution composed of synthesized nanoparticles.
 3. The method according to claim 1, wherein the nanoparticles are semiconductor nanoparticles selected from the group consisting of HgTe, HgSe, HgS, CdTe, CdSe, CdS, ZnTe, ZnSe, ZnS, PbTe, PbSe, PbS and mixtures thereof.
 4. The method according to claim 1, wherein the organic material is selected from the group consisting of alkyldithiol, mercaptoalkyl alcohol, mercaptoalkyl acid, and mixtures thereof.
 5. The method according to claim 1, wherein the organic material is selected from the group consisting of 1-thioglycerol, hexanedithiol, octanedithiol, nonanedithiol, dodecanedithiol, mercaptohexanol, mercaptooctanol, mercapto acetic acid, mercapto propionic acid, and mixtures thereof.
 6. The method according to claim 1, wherein the nanoparticles have at least one of single structures and core-shell structures.
 7. The method according to claim 1, wherein the insulating film is selected from the group consisting of SiO₂, Al₂O₃, Indium Tin Oxide and HfO.
 8. The method according to claim 1, wherein the electrodes are selected from the group consisting of Au, Al, Pt and Cu.
 9. The method according to claim 1, wherein the nanoparticles are uniformly distributed in close proximity to each other and have a uniform size. 